For conventional polycrystalline diamond used for such tools as a cutting bit, a dresser and a die as well as a drill bit, an iron-group element metal such as Fe, Co and Ni, carbonate such as CaCO3, and the like are employed as a sintering agent for promoting sintering of a source material, and ceramics such as SiC and the like are used as a binder for binding source materials.
Polycrystalline diamond above is obtained by sintering diamond powders, which are source materials, together with a sintering agent at a high-pressure and high-temperature condition (generally, the pressure being around 5 to 8 GPa and the temperature being around 1300 to 2200° C.) at which diamond is thermodynamically stable.
Polycrystalline diamond thus obtained contains the used sintering agent therein. Such a sintering agent has no small effects on such mechanical characteristics as hardness and strength and on heat resistance of polycrystalline diamond.
Polycrystalline diamond from which the sintering agent above has been removed by acid treatment and sintered diamond excellent in heat resistance for which heat-resistant SiC has been used as a binder have also been known, however, they are low in hardness and strength and insufficient in mechanical characteristics as a tool material.
Meanwhile, a non-diamond carbon material such as graphite, glassy carbon or amorphous carbon can directly be converted to diamond at an ultra-high pressure and temperature, without using a sintering agent or the like. By directly converting a non-diamond phase to a diamond phase and simultaneously carrying out sintering, polycrystalline single-phase diamond is obtained.
F. P. Bundy, J. Chem. Phys., 38 (1963) pp. 631-643 (NPL 1), M. Wakatsuki, K. Ichinose, T. Aoki, Japan. J. Appl. Phys., 11 (1972) pp. 578-590 (NPL 2), and S. Naka, K. Horii, Y. Takeda, T. Hanawa, Nature, 259 (1976) p. 38 (NPL3) disclose polycrystalline diamond obtained by direct conversion of graphite serving as a source material at such an ultra-high pressure from 14 GPa to 18 GPa and an ultra-high temperature of 3000K or higher.
Each polycrystalline diamond above, however, is produced by direct electrical heating in which a conductive non-diamond carbon material such as graphite is heated by directly feeding a current therethrough, and hence unconverted graphite inevitably remains. In addition, a particle size of diamond is non-uniform and sintering tends to be partially insufficient. Therefore, such mechanical characteristics as hardness and strength are not sufficiently high and only a piece-like polycrystal is obtained, and hence practical use has not been achieved.
T. Irifune, H. Sumiya, “New Diamond and Frontier Carbon Technology,” 14 (2004) p. 313 (NPL 4) and Sumiya, Irifune, SEI Technical Review, 165 (2004) p. 68 (NPL 5) disclose a method of obtaining dense and high-purity polycrystalline diamond by direct conversion and sintering by indirect heating at an ultra-high pressure not lower than 12 GPa and an ultra-high temperature not lower than 2200° C., with the use of high-purity, highly-crystalline graphite as a starting material. Though diamond obtained with this method has very high hardness, its practical characteristics such as wear resistance, chipping resistance, and resistance to crack propagation have been insufficient and unstable.
Naturally produced polycrystalline diamonds (carbonado, ballas and the like) have also been known and some are used for a drill bit. On the other hand, variation in material is great and yield is also small, and thus they are not much industrially used.
Depending on some applications, single-crystal diamond is used. Use thereof, however, is limited to use for an ultra sophisticated tool or a precision wear-resistant tool due to restrictions in terms of dimension and cost, and thus applications and conditions for use are restricted by cleavability and anisotropy in mechanical characteristics of single-crystal diamond.